This invention relates to methods and apparatus for removing oil base drilling mud or like contaminants from drill cuttings and, more particularly, it concerns a system and method which combines the use of centrifugal, cyclonic, filtration and gravitational separation techniques and which incorporates a continuous phase liquid, such as base oil or salt water with surfactants, which washes the contaminants from the cuttings, is separated from the contaminants, and is recycled through the system.
A variety of systems and techniques have been developed to clean oil or oil-based drilling mud from drill cuttings in order to provide for an environmentally safe disposal of the cuttings. Recently, there has been a great deal of activity directed toward development of a practical system capable of cleaning contaminated drill cuttings at an offshore location so as to allow for disposal of the cleaned cuttings directly into the ocean. Without such a system for cleaning contaminated cuttings, the offshore use of oil muds is a very expensive proposition since environmental regulations require that oil-contaminated, drilled cuttings be hauled from the offshore rig to an onshore disposal facility.
One approach to cleaning oil contaminated or coated cuttings is to burn the oil off the cuttings using, for example, very high temperature heat lamps or steam. Examples of this approach are disclosed in U.S. Pat. Nos. 4,209,381, issued on June 24, 1980; 4,595,422, issued on June 17, 1986; and 4,683,963, issued on Aug. 4, 1987. Such systems for burning off the oil from the cuttings suffer from drawbacks, such as, insurance problems due to the questionable use of high heat on an offshore rig and only a partial cleaning of the cuttings caused by an unequal heating of the contaminated cuttings.
Another and more practical approach to the offshore cleaning of oil contaminated cuttings is to wash the cuttings with a detergent solution, separate the cuttings from the mixture of wash solution and oil, and then discharge the cleaned cuttings into the ocean. U.S. Pat. Nos. 3,688,781 issued on Sept. 5, 1972; 3,693,733, issued on Sept. 26, 1972; and 4,546,783, issued on Oct. 15, 1985 disclose examples of such washing systems or associated methods. These conventional washing systems are less than desirable because they do not provide the necessary cleaning, the washing solution itself may pose a threat to the environment, or they require periodic shutting down of the system to allow for the settling of oily particles or for the removal of a highly contaminated washing solution which must be hauled to an onshore disposal site.
Yet another approach to cleaning oil contaminated drill cuttings involves the use of specialized solvents which are usually miscible with oil but essentially immiscible with water and which may be in liquid form during one stage of the cleaning process and in vapor form during another stage of the process. For example, U.S. Pat. No. 4,836,302 discloses a complex apparatus for removing and recovering oil and other oil-based drilling mud additives from drill cuttings using an easily vaporized solvent, such as trichlorotrifluoethane. Such a complex separation system is undesirable not only from the standpoint of unit cost, but also high operating costs and problems associated with the use of volatile and/or environmentally dangerous solvents. Other examples of specialized cleaning solvents are disclosed in U.S. Pat. Nos. 4,040,866, issued on Aug. 9, 1977, and 4,645,608 issued on Feb. 24, 1987.
In light of the above, there is a need for an improved system for thoroughly and safely cleaning oil or oil mud contaminated drill cuttings prior to disposal.